In recent years, miniaturization and high functionalization of mobile devices such as portable phones and PDAs (Personal Digital Assistants) have progressed. For mounting technologies that can cope with such trends, mounting structures such as BGAs (Ball Grid Arrays) and CSPs (Chip Scale Packages) are frequently used. Mobile devices are likely to be exposed to mechanical load such as drop impact. QFPs (Quad Flat Packages) absorb shocks on its lead part. However, in BGAs and CSPs, which have no lead parts for cushioning shocks, it has been increasingly important to secure reliability in the impact resistance.
The melting point of Pb eutectic solder, which is conventional typical solder, is 183° C. However, with regard to the melting point of recent lead-free solder, the melting point of typical Ag—Sn—Cu solder is about 30° C. higher than the PB eutectic solder. The maximum temperature in the profile of the reflow furnace is increased to 220° C. to 260° C.
Therefore, in cases where components with insufficient high-temperature resistance are mounted on wiring substrates, only such components are joined to the substances by spot soldering in a separate step. Consequently, the productivity is significantly low.
For that reason, low-melting-temperature Pb-free solder, such as Sn—Zn solder, Sn—Ag—In solder, and Sn—Bi solder, which is free of the demerit of the high melting point of the Sn—Ag—Cu solder, has been started to be used. However, with regard to the BGA connection using the Sn—Zn, Sn—Ag—In or Sn—Bi solder, connection reliability in the soldered connection parts is still unclear.
As a countermeasure, a production method in which a solder paste using a thermosetting resin as a flux has been proposed in order to enhance reliability in the impact resistance in connection parts (Japanese Patent No. 5,204,241, Publication)
In this method, the resin in the solder paste is separated from the solder when soldering is conducted, and the resin covers the periphery of the solder. Thus, the strength of the solder is reinforced by the resin.